In physical geography, a dune is a hill of loose sand built by the wind or the flow of water. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Dunes occur, for example, in some deserts, and along some coasts.